Characterization of porous materials by Focused Ion Beam Nano-Tomography
Asphalt internal structure characterization with X-Ray computed tomography
1. Asphalt internal structure
characterization with X-Ray
computed tomography
Denis Jelagin, Ibrahim Onifade, Alvaro Guarin and Nicole Kringos
KTH, Highway and Railway Engineering
2. Outline
Understanding of asphalt mixture
properties based on constituent
materials spatial distribution and their
mechanical properties:
- Determination of quantitative parameters
to describe asphalt internal structure.
- Mechanical modeling with finite element
method to quantify the impact the
constituent material parameters have on
mixture mechanical behavior
3. Asphalt mixture internal structure and
its effect on field performance
Asphalt consists of three main phases: stones, binder
and air voids; their spatial distribution and properties
have a major impact on asphalt performance:
• Stones and stone-to-stone contacts provide a primary
load carrying mechanism in compression and shear,
especially at high temperatures
• Bitumen-based binder and its distribution control
tensile stiffness and fracture resistance
• Air void structure controls mixture permeability,
resistance to bleeding and ageing
Deficient internal structure of asphalt results in
pavement failures
5. X-Ray computed tomography (CT)
characterization of asphalt
X-Ray CT system to acquire images Avizo® Fire to segment CT data and to obtain
with spatial resolution of 5-100 µm quantitative parameters for specimens structure
Preprocess
for FEA
Use mechanical testing to investigate FEM modeling to quantify the
the impact of the observed internal effect of different micromechanical
structure on materials performance and geometrical parameters on
materials performance
6. X-Ray CT characterization of asphalt
• Porous (“quiet”) asphalt
- Cylindrical core 80 mm high x 100 mm
diameter
- High air voids (20%) to facilitate drainage
and noise damping
• CT data with 59x59x59 µm voxel size is
acquired
• Analysis is performed on a rectangular
volume (60x60x40 mm) in the center of
the specimen
7. Analysis procedure
X-Ray CT slice before
post-processing:
• Significant density
variation within phases
(stones and binder)
• Considerable amount of
beam hardening
• Image noise
8. Analysis procedure
Corrected image:
• Histogram equalization to
improve contrast
• Noise reduced with median
filter (3x3 kernel) and edge
preserving smoothing filter
• Beam hardening corrected
based on background flat field
correction
- Illumination profile:
9. Analysis procedure
Segmented image:
• Phase (air voids and
stones) identification with
threshold-based
segmentation
• Binder is defined as the
difference between total
volume, stones and air
voids
• Stones are separated based
on the distance map with
watershed segmentation
• Stones smaller than 2.34
mm are filtered out and
replaced with binder
10. Results
Stone skeleton
Reconstructed stone surfaces
Parameters describing stone size
distribution, their shape , roughness and
orientation in the material are obtained.
These parameters define to a great
extent the stone skeleton strength and
its susceptibility to aggregate breakage.
12. Results
Stone skeleton (contact regions)
During separation based on
the distance map, the contact
regions between stones are
identified:
• Regions where the separation
lines intersect with the
segmented stone phase
represent contact regions
• A sensitivity range for
contact detection is defined
presently as 108 µm (2
pixels)
13. Results
Stone skeleton (contact regions)
The stone contact regions provide a
primary load transferring mechanism in
compression and shear.
In several recent studies contact zones
geometry and orientation have been
correlated with asphalt compactability
and rutting performance.
14. Results
Air voids
Reconstructed air voids surfaces
CT data is analyzed in order to evaluate if
the air void distribution and connectivity
in the specimen agree with the design
parameters of the mixture.
Reduced air void content at the bottom of
the specimen results in compromised
permeability and noise damping
capabilities.
15. Micromechanical analysis with FEM
FEM simulations based on structural information obtained with
the X-Ray CT allow to:
• Improve our understanding of the mechanical behavior of asphalt
and its degradation processes.
• Quantify the effect of using constituent materials with improved (or
worsened) characteristics.
• Develop a “virtual specimen” type of approach for asphalt mixture
design. This will provide a cost effective way to optimize different
asphalt mixture parameters, e.g. binder type, air void contents and
stone size distribution for better field performance.
Analysis results illustrate the capability of this method to
capture stress concentrations and strain localization arising
due to differences in mechanical and thermal properties of the
phases.
16. Uniaxial tension and thermal stresses(2D)
h=0.1 mm
• Reconstructed surfaces and
volumes are exported to
COMSOL Multiphysics package
• Mechanical and thermal
properties representative for
each phase are assigned to
stone and binder regions in the
model
• 2D plane strain analysis for:
- Uniaxial tension
- Thermally induced stresses
(temperature at the air void
boundary is reduced at a rate of
10ºC/hour)
17. Uniaxial tension (2D)
• Strains are localized in the
binder phase
• Strains up to 12% are observed
as compared to approx. 0.2%
predicted for homogeneous
material case
• The information obtained with
this type of modeling can be
used to identify representative
stress and strain levels for
binder testing
18. Uniaxial tension (2D)
• Load transfering chains
can be seen in the
material
• Only main load
transfering regions in
the binder are subjected
to a tensile stress
>10MPa (as compared
to the uniform tension of
19 MPa for the uniform
material case)
19. Thermal stresses (2D)
• Temperature variation of
approx. 1.5ºK can be seen.
The temperature gradient
would increase with
increasing cooling speed and
decreasing air void content.
• As the specimen is not
constrained, this type of
thermal loading would result
only in negligible stresses in
the homogeneous material.
20. Thermal stresses (2D)
• Stones are subjected to
higher stresses due to their
higher stiffness
• Regions of localized tension
are formed in the binder due
to difference in thermal
contraction properties
between phases.
• Maximum tensile stresses in
the binder reach approx. 2.5
MPa
21. Uniaxial compression (3D)
Analysis of small regions
around stone-to-stone
contact zones to get insight
into the local degradation
mechanisms:
- Work in progress…
22. Uniaxial compression (3D)
Von Mises stress localized in stones around
contact points
Understanding the mechanisms controlling:
• stone breakage and polishing during asphalt
compaction
• Micro-fracture initiation in binder films
Compressive strains localized in the binder